1. Field of the Invention
The present invention relates generally to disk drive head switching and track following servo operation and more specifically relates to methods and structures for assuring that switching from a first head to a second head cannot leave the second head in a “zipper zone” of its corresponding surface to thereby lose track following synchronization.
2. Discussion of Related Art
Computing storage subsystems generally utilize rotating, magnetic, disk storage devices for rapid, random access to large volumes of stored data. In general, stored information on a rotating magnetic disk drive comprises magnetic flux density changes recorded around the circumference of concentric tracks positioned radially from the inner diameter (ID) of a disk media recording surface to the outer diameter (OD) of a disk media surface. Present day disk drives typically include a plurality of such recording media stacked on a common rotating spindle and each with a corresponding read/write head for recording information on the corresponding magnetic recording surface and for sensing previously recorded data therefrom. The plurality of read/write heads are typically arranged on actuator arms. A plurality of such arms are typically arranged in a comb-like structure between the various disk recording surfaces such that a read/write head is “flying” adjacent its corresponding recording media surface. The comb-like structure is actuated so that all read/write heads may be radially positioned to a particular concentric track. The read/write head associated with each disk drive recording media surface is therefore moved in unison with all other read/write heads. In other words, each read write head is positioned over substantially the same concentric track position—each on its corresponding magnetic recording surface. Though each of the heads of the comb-like structure may be roughly positioned to a common radial track position, the head position must be finely tuned while in use to more accurately position the head for reading or writing on the desired track. Servo control information may be pre-written on the disk media recording surface at various points about the circumference of each track to identify the track position of each concentric track (e.g., the track number and other relevant information regarding the track). The servo control information is read from the head and processed to allow a servo control mechanism to finely tune the radial position of the head relative to the center of the track as data is being read or written by the head on that track. To allow this fine tuning of radial position based on recorded servo information on each track, only one head is used at a time and the disk drive controller logic switches between multiple heads as needed to read or write the requested blocks of data.
It is common in disk storage systems to switch from one recording head to another recording head while reading or writing to avoid time consuming mechanical delays (latency) associated with radially repositioning the comb-like structure of multiple read/write heads and the delay in allowing the mechanism to settle at its new radial position. Switching from one head to another head incurs no such mechanical latency but rather merely incurs electronic switching latency and any requisite processing delays associated with re-synchronizing servo and timing information recorded on each concentric track and used for track following servo control.
Servo information is typically pre-written on disk drive media surfaces during the manufacturing process to accurately place the servo information that defines the radial position of each track thereon. Though the servo information is carefully and precisely placed on the multiple disk surfaces during the manufacturing process, mechanical and environmental conditions may change slightly over time. Thus, as a disk drive controller switches from one head to another, some delay is incurred in tuning the precise position of the next head based on the servo information recorded on its corresponding track position.
Ever increasing demands for storage capacity have led disk drive manufacturers to seek ever higher storage densities. Storage density is typically measured both linearly and radially. Linear density is measured as bits per inch (“BPI”) in the longitudinal direction of a recorded track (i.e., in the direction in which the head flies over the rotating disk media). Radial density is measured in the radial direction as tracks per inch (“TPI”). A real density is a common measurement of the total storage density of a disk drive and is determined as the product of linear density (BPI) and radial density (TPI).
Numerous well-known techniques have been applied to increase in the linear density recorded around the circumference of each concentric track. Some such techniques involve simple encoding algorithms to increase the linear density while maintaining high reliability. In so-called longitudinal recording, magnetic flux changes are recorded “longitudinally” (i.e., substantially parallel to the direction of the magnetic surface rotating under the read write ahead). In “vertical” or “perpendicular” recording techniques, magnetic flux changes are recorded substantially perpendicular to the longitudinal direction of the head/media movement. Vertical or perpendicular recording provides a higher linear density of storage.
Other techniques are applied to increase the radial density (TPI). In the radial direction, the distance between the center of a recorded track and the center of an adjacent recorded track (“track pitch”) tends to be larger than the width required for a successful reading. Placement of the servo information written at time of manufacture determines the center position of each track. Placing the servo information closer for each track increases the radial density. Spacing the tracks further apart than the minimum distance allows tracks to be more easily randomly re-written though the newly written (or re-written) information may be slightly displaced relative to older information previously at the same track. The magnetic flux changes representing one track are far enough from a next adjacent track that one will not interfere with reading of the other. If an adjacent track were recorded too close to another track, the sensing by the read head may receive interference sensed as noise in reading the intended track. Thus the signal to noise ratio of the data to be read may be reduced. By keeping the tracks further apart to allow for random re-writing at different times and under different conditions, the interference between tracks is reduced and the signal to noise ratio for data to be read is maintained at a higher level.
One technique generally known in the disk drive art for increasing radial density in certain storage applications is to record information in a streaming manner from one track to a next adjacent track in sequential order. Where data is so recorded sequentially with no need to randomly re-write individual tracks, the recorded data may be applied to the magnetic media at a very high radial density by densely packing the recorded information such that a next sequential track abuts or even partially overwrites an earlier recorded track. Such an operation is often referred to as “trimming” in that it trims the track pitch to nearly the minimum required for accurately sensing data. Such techniques are more readily applied in vertical or perpendicular recording but may also be applied in longitudinal recording. Key to such applications is that the data is written in a substantially continuous, sequential stream and read in a substantially continuous, sequential stream over the entire recorded area of the disk drive. In such applications the mechanical and environmental parameters that may affect track positioning and following controls will not change substantially during the continuous read or write sequence. Thus, track following servo controls may more reliably follow the center position of each concentric track and allow for closer spacing of the concentric tracks. Numerous disk storage applications may utilize such sequential writing/reading techniques such as audio/video capture or presentation and other forms of streamed data capture and presentation.
Where such overlap and trim techniques are used in recording data at a higher radial density, it is also generally known to record the concentric tracks in each of two radial directions. In other words the disk surface is written bi-directionally—a first portion of the disk is recorded on sequential tracks starting from the inner diameter (ID) of a recording surface moving toward a middle diameter (MD) and a second portion of the disk drive is recorded on sequential tracks starting from an outer diameter (OD) to the middle diameter (MD). The middle diameter (MD) is in general selected as a position where the read/write head is at a minimally skewed angle with respect to the tangent line of recording on the circular track. As the read/write head is radially positioned more inward or more outward, the geometry of the actuator arm and head mounted thereon changes a bias angle imposed between the angle of the write head and the concentric track on which it records. Typically, disk drive manufacturers try to configure the heads and actuator arms so that the bias angle is minimized at a center point in the radial travel of the head and hence averaged over the entire radial range of head travel. The middle diameter (MD) is therefore often referred to as the “zero skew” position.
Where the two directions of writing join at approximately the MD concentric track position, the overlap and trim features of the high radial density writing may render one or more tracks near MD unusable. This area is sometimes referred to as a “zipper track” or “zipper zone”. As used herein, both terms refer to a zone of multiple tracks near the MD track location on each surface potentially unusable because of the overlap in bi-directional writing.
A problem arises where such bi-directional sequential writing generates such a zipper zone with multiple surfaces and associated multiple read/write heads. Track following servo mechanisms associated with a read/write head may be used to avoid the zipper zone on any surface of the disk drive. If the read/write head were allowed to enter the zipper zone, overlapped writing may render servo information useless or unreadable and hence lose synchronization for track following servo mechanisms. Therefore, control mechanisms of such a disk drive utilizing bi-directional high radial density writing carefully avoid the zipper zone on each disk surface.
However, when it is desired to switch from one head to another head in a multiple surface disk drive, misalignment of track positioning between multiple read/write heads may cause a head switch operation to switch to a second read/write head that is presently physically positioned within the zipper zone of its corresponding disk surface. In other words, though a first head may be outside its corresponding zipper zone and the track following servo controls properly functioning, the electronic switch to the second head could occur while the second head (misaligned relative to the first head) is positioned within the zipper zone of its corresponding disk surface. The two surfaces may be slightly misaligned with respect to one another due to mechanical or environmental condition changes over time in the disk drive. When such an event occurs, track following servo mechanisms may lose synchronization because the second read/write head attempts to use servo information that has been improperly overwritten or is otherwise unreadable due to its location within the zipper zone. Recovery from such a track following servo error can be a time consuming event in disk drive control and hence can diminish disk drive performance.
It is evident from the above discussion that improved head switching techniques and structures are needed where a disk drive uses bi-directional, high radial density track writing techniques that generate an unusable zipper track or a zipper zone on each of multiple disk surfaces.